The present invention relates to vehicle test equipment and, in particular, to chassis dynamometers, and also relates to vehicle weighing systems.
Chassis dynamometers are used for road simulation and other controlled engine loading tests for the purpose of exhaust emission measurement, vehicle performance testing and diagnostics. In order to meet the requirements of the 1990 Clean Air Act there has been an increasing use of chassis dynamometers in vehicle emissions testing. Dynamometers provide a better simulation of actual driving conditions and, as a consequence, cause the vehicle to generate more realistic levels of exhaust emissions, than do tests in which the vehicle's engine is not loaded. Test vehicles transfer their power to the dynamometer through a frictional drive assembly, such as a set of rollers rotating under the vehicle driving wheels. The rollers are mechanically coupled to a power absorber that applies the designated loading by means of computer control. Inertia simulation is typically achieved mechanically or electromechanically.
Dynamometers are commonly provided with a lift mechanism, which may include air bags connected to lift plates for engaging the vehicle tires and raising them above the levels of the rollers to facilitate moving the vehicle onto and off of the roller assemblies.
The engagement of the vehicle tires on the rollers creates friction losses, which are proportional to the vehicle weight supported on the dynamometer rollers. In order to obtain accurate measurements, the vehicle must be weighed to calculate the friction losses to the tires. Thus, certain state regulations now require that the "axle weight" of the vehicle, i.e., the weight of the part of the vehicle which actually rests on the dynamometer, must be measured, this axle weight usually being some predetermined percentage of the overall vehicle weight. Thus, it is necessary for testing facilities to provide some means for measuring the vehicle axle weight. One such technique is to use strain gauges for measuring the vehicle weight before the vehicle is placed on the rollers. A strain gauge generates a voltage proportional to the weight, but such strain gauge weight measuring systems are rather expensive.